A very abundant poly A containing RNA is complementary to a family of nearly identical Dm segments which we call copia (Young and Hogness, 1977; Finnegan, Rubin, Young and Hogness, 1978). An intriguing property of the copia sequence involves its repetition; copia is repeated in a dispersed fashion so that copies of the gene are found at about 30 chromosomal sites scattered throughout the genome (Young and Hogness, 1977). Furthermore, this arrangement is polymorphic; different wild type stocks of Drosophila show different arrangements of these genes (Rubin, Backner, Brorein, Potter and Strobel, unpublished), and the arrangements in different strains tend to be non-overlapping (Young, unpublished). RNA complementary to copia genes is found in two size classes; 2.1 kb, and 5.3 kb, (Carlson and Brutlag, 1978; Young and Hogness, in preparation). The two RNAs are transcribed in the same direction and the sequences found in the smaller RNA are for the most part, contained within the larger RNA (Schwartz and Young, unpublished). The following questions will be addressed: (1) Is either the 2.1 kb or 5.3 kb RNA translated and what is the precise relationship of these two RNAs to each other and to the copia genes? (2) What is the effect of the genetic deletion of one or several copies of copia? Will these deficiencies be compensated because copia is repeated? (3) How do copia genes change position and how many chromosomal positions can copia occupy? (4) What fraction of the genomic DNA is tied up in transposable sequences like copia?